Sunan Ibn Majah 1557

It was narrated that Anas bin Malik said:“When the Prophet (ﷺ) died, there was a man in Al-Madinah who used to make a niche in the grave and another who used to dig graves without a niche. They said: ‘Let us pray Istikharah to our Lord and call for them both, and whichever of them comes first, we will let him do it.’ So they were both sent for, and the one who used to make the niche-grave came first, so they made a niche-grave for the Prophet (ﷺ).”