This question can be studied in two parts:
1. Did Muhammad bin Uthman Umari meet with the Imam of Time ('atf) during Hajj season?
2.  Muhammad bin Uthman Umari has been reported to have said that the Imam of Time ('atf) attends the Hajj ceremony every year and that one can meet with the Imam during Hajj season. How far is this report true? We shall deal with both questions here:
A) Meeting with the Imam of Time ('atf) during the period of occultation:
It is possible to meet with the Imam of Time ('atf) during the period of occultation. Many scholars who is believed to be honest and just ('adil) and who can be trusted in such matters have narrated stories of people meeting with Imam Mahdi, may Allah hasten his reappearance. These stories can be found in their books along with names of people who have been blessed with the opportunity to meet with him. Such meetings have taken place both during the period of minor occultation and major occultation. [1]
During the period of minor occultation, the Imam of Time ('atf) communicated with people through his special deputies whom he had appointed as intermediaries. The four deputies represented the Imam and acted as his deputies one after another. Throughout the period of minor occultation, these deputies met with the Imam and also some people other than the deputies managed to meet with the Imam sometimes.
The second special deputy of the Imam of was Muhammad bin Uthman Umari who, after his father Uthman bin Sa'eid, undertook the position of deputyship at the behest of the Imam. He was blessed with the opportunity to be the Imam's emissary for nearly forty years. Obviously, Muhammad bin Uthman met with the Imam throughout this period. He was an intermediary between people and the Imam; because deputyship requires one to be constantly in contact with the Imam and to facilitate communication between him and other people.
B) As for whether Muhammad bin Uthman has met the Imam of Time ('atf) during Hajj ceremony or not, there is a report from Muhammad bin Uthman himself who says that he was asked by Abdullah bin Ja'far al-Humayri whether or not he had seen the Imam of Time and he had replied, "It was in the Sacred House of Allah when I last saw the Imam whilst he was praying and saying: "'O Lord, fulfill what you have promised me."[2]
Also, Muhammad bin Uthman says that he saw the Imam holding the curtain of the Ka'abah and saying: "O Lord, take revenge from your enemy."[3] This saying of Muhammad bin Uthman signifies that he had seen the Imam (AS) beside the Ka'abah but it does not indicate whether he had seen him during Hajj season or some other time.
Muhammad bin Uthman has been reported to have said: "By Allah, the Imam of Time ('atf) attends the Hajj ceremony every year. He sees people and knows them; people also see him but they do not recognize him."[4]
There are proofs and evidence confirming Muhammad bin Uthman's saying:
1. Imam Sadiq, peace be upon him says, "People lose their Imam; he attends the hajj ceremony during hajj season and he sees people but people do not see him."[5]
2. One of the best acts of worship which happens only once a year is the obligation of Hajj which is performed by able-bodied and financially capable Muslims. Performing hajj earn the pilgrim a lot of rewards and it has been highly recommended; its spiritual benefits are numerous and that is the reason why Hajj has been considered to be one of the pillars of Islam. Imam Baqir, peace be upon him, says: "Islam is founded upon five pillars: prayer, Zakat, fasting, Hajj and Wilayah."[6]
The Imams (AS) advised Shiites to perform Hajj every year, if they could. They were advised to attend the Hajj ceremony frequently.  In a tradition narrated from Imam Sadiq (AS), the Imam says to his companion: "O 'Isa, if you could find some bread and salt to eat (that is to say, if you could find enough to eat during the year), then perform Hajj every year."[7]  In another tradition, the Imam has been reported to have said: "Perform pilgrimage of the House of Allah, do it persistently, because constant performance of Hajj will repel hardship from you."[8] In this narration, a "persistent performer of Hajj" (i.e. one who performs Hajj every year) has been praised.[9]
We also see in the normative conduct of the imams (AS) that they endeavored to perform Hajj as many times as they could; they made sure not to miss Hajj. For instance, Imam Hasan Mujtaba, peace be upon him, has said that he travelled to Makkah on foot more than twenty times to perform Hajj.
In jurisprudential books also, the jurisprudents have dedicated a special chapter on Hajj and desirability of performing it every year.[10]
It is unlikely of the Imam of Age (AS) to lose this virtue by not acting upon what his noble forefathers commended. It can, therefore, be said that one of the places where the Imam visits very often especially during the Hajj season is the Ka'abah, or the House of Allah.
3. It is inferred from narrations that the Imam of Age, may Allah hasten his reappearance, is not living far away from people and society. In fact, he is living with people and in their midst. He attends public congregations but people do not recognize him. One of the important congregations in Islam is the great Hajj congress which is attended most likely by the Imam of Time, Imam Mahdi, may Allah hasten his reappearance.

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