Sayyiduna Thawban (may Allah be pleased with him), the freed slave of the Messenger of Allah (peace and blessings be upon him), narrates: I was sitting with the Messenger of Allah (peace and blessings be upon him) when one of the rabbis of the Jews came and said: As-salamu alayka (peace be upon you), O Muhammad! I pushed him so hard that he nearly fell. He said: Why are you pushing me? I said: Do you not say, 'O Messenger of Allah'? The Jew said: We only call him by the name his family gave him. The Messenger of Allah (peace and blessings be upon him) said: "My name is Muhammad, which my family gave me." The Jew said: I have come to ask you. The Messenger of Allah (peace and blessings be upon him) said to him: "Will it benefit you at all if I tell you?" He said: I will listen with my ears. He (peace and blessings be upon him) made marks on the ground with a stick he had with him and said: "Ask." The Jew said: On the day when the earth is changed into another earth, and the heavens as well, where will the people be? The Messenger of Allah (peace and blessings be upon him) said: "They will be in darkness before the Bridge (al-Sirat)." He said: Then who will be the first to cross? He (peace and blessings be upon him) said: "The poor among the Muhajirun." The Jew said: When they enter Paradise, what will their gift be? He (peace and blessings be upon him) said: "The extra lobe of the liver of the fish." He said: What will their food be after that? He (peace and blessings be upon him) said: "The bull of Paradise, which grazes on its edges, will be slaughtered for them." He said: What will their drink be with it? He (peace and blessings be upon him) said: "From a spring called Salsabil." He said: You have spoken the truth. And I have come to ask you about something which no one among the people of the earth knows except a prophet, or one or two men. He (peace and blessings be upon him) said: "Will it benefit you if I tell you?" He said: I will listen with my ears. He said: I have come to ask you about the child. He (peace and blessings be upon him) said: "The man's fluid is white and the woman's fluid is yellow. When the two come together, if the man's fluid prevails over the woman's fluid, it is a boy by the permission of Allah; and if the woman's fluid prevails over the man's fluid, it is a girl by the permission of Allah." The Jew said: You have spoken the truth, and indeed you are a prophet. Then he turned and went away. Thereupon the Messenger of Allah (peace and blessings be upon him) said: "He asked me about what this one was asking me, and I had no knowledge of any of it until Allah, the Mighty and Majestic, informed me."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1401