According to Hadiths and Sayings of Prophet on issues of women in “Al-Jami Al-Saghir” written by Jalal ul-Din Al-Suyuti.

Narrated by Ibn Abbas: “I (Muhammad) put on her my shirt that she may wear the clothes of heaven, and SLEPT with her in her coffin.”

This is from a book called "Kanz Al Umal" (The Treasure of the Workers), in the chapter of "The issues of women", authored by Ali Ibn Husam Aldin, commonly known as Al-Mutaki Al-Hindi. He based his book on the hadiths and sayings listed in "Al-Jami Al-Saghir," written by Jalal ul-Din Al-Suyuti.

Narrated by Ibn Abbas:

"I (Muhammad) put on her my shirt that she may wear the clothes of heaven, and I SLEPT with her in her coffin (grave) that I may lessen the pressure of the grave. She was the best of Allah’s creatures to me after Abu Talib"… The prophet was referring to Fatima , the mother of Ali.

The Arabic scholar Demetrius explains : "The Arabic word used here for "slept" is "Id'tajat," and literally means "lay down" with her. It is often used to mean, "lay down to have sex." Muhammad is understood as saying that because he slept with her she has become like a wife to him so she will be considered like a "mother of the believers." This will supposedly prevent her from being tormented in the grave, since Muslims believed that as people wait for the Judgment Day they will be tormented in the grave. "Reduce the pressure" here means that the torment won't be as much because she is now a "mother of the believers" after Muhammad slept with her and "consummated" the union."