Sayyiduna Ibn Abbas (may Allah be pleased with them both) used to narrate that Abu Bakr (may Allah be pleased with him) uncovered the blessed face of the Messenger of Allah (peace and blessings be upon him), which was covered with a cloth; then he looked at the face of the Prophet (peace and blessings be upon him), then bent over and kissed it.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10677