Abdullah bin al-Harith relates that Sayyiduna Jundub (may Allah be pleased with him) heard the Prophet (peace and blessings be upon him) say, five days before his (peace and blessings be upon him) death: "I have had brothers and friends among you, and I declare myself free before Allah of having any khalil (intimate friend) from among you. Had I taken a khalil from my Ummah, I would have taken Abu Bakr (may Allah be pleased with him) as a khalil. And indeed my Lord, the Mighty and Majestic, has taken me as a khalil just as He took Ibrahim (peace be upon him) as a khalil. Beware! Those before you used to take the graves of their prophets and righteous people as places of prostration. So do not take graves as places of prostration; I forbid you from that."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1673