Sahih Muslim Book 31, Hadith Number 6004.

‘Aisha reported: We, the wives of Allah’s Apostle (may peace be upon him), were with him (during his last illness) and none was absent there-from that Fatima, who walked after the style of Allah’s Messenger (may peace be upon him), came there, and when he saw her he welcomed her saying: You are welcome, my daughter. He their made her sit on his right side or on his left side. Then he said something secretly to her and she wept bitterly and when he found her (plunged) in grief he said to her something secretly for the second time and she laughed. I (‘Aisha) said to her: Allah’s Messenger has singled you amongst the women (of the family) for talking (to you something secretly) and you wept. When Allah’s Messenger (may peace be upon him) recovered from illness, I said to her. What did Allah’s Messenger (may peace be upon him) say to you? Thereupon she said: I am not going to disclose the secret of Allah’s Messenger (may peace be upon him). When Allah’s Messenger (may peace be upon him) died, I said to her: I adjure you by the right that I have upon you that you should narrate to me what Allah’s Messenger (may peace be upon him) said to you. She said: Yes, now I can do that (so listen to it). When he talked to me secretly for the first time he informed me that Gabirel was in the habit of reciting the Qur’an along with him once or twice every year, but this year it had been twice and so he perceived his death quite near, so fear Allah and be patient (and he told me) that he would be a befitting forerunner for me and so I wept as you saw me. And when he saw me in grief he talked to me secretly for the second time and said: Fatima, are you not pleased that you should be at the head of the believing women or the head of this Umma? I laughed and it was that laughter which you saw.