Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: When Sayyidah Fatimah (may Allah be pleased with her), the mother of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him), passed away, the Prophet (peace and blessings be upon him) took off his shirt and gave it for her to be dressed in, and he himself lay down in the grave of Sayyidah Fatimah (may Allah be pleased with her). When the soil had been levelled over her grave, someone said: O Messenger of Allah! We saw you do something that you have not done for anyone. the Prophet (peace and blessings be upon him) said: "I gave her my shirt to wear so that she may wear the clothing of Paradise, and I lay down in her grave so that the squeezing of the grave would be lightened for her; for, after Abu Talib, she treated me the best of all Allah's creation."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat; its chain contains a narrator, Sa'dan bin al-Walid, whom I do not know. The remaining narrators of this narration are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 15400