Musnad Abu Ya'la al-Mawsili 150:380

'Abdullah ibn Muti' narrated to us, Hushaym narrated to us, from al-'Awwam, from the one who narrated to him, from 'A'ishah, she said: When the Messenger of Allah (peace and blessings of Allah be upon him) laid the foundation for the mosque of Medina, he brought a stone and placed it. Abu Bakr came with a stone and placed it, and 'Umar came with a stone and placed it, and 'Uthman came with a stone and placed it. She said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about that, and he said: "This is the matter of the Caliphate after me."