Hadith: Abu Bakr entered upon the Messenger of Allah (may Allah’s peace and blessings be upon him) after his death. He put his mouth between his eyes, put his hands on his temples, and said: O my Prophet, O my ‎close friend, O my bosom friend

عن عائشة رضي الله عنها ، أنَّ أبا بكر دَخَل على النبي صلى الله عليه وسلم بعد وفاته، فَوَضَع فَمَهُ بَيْن عيْنَيْه، ووَضَعَ يديْه على صُدْغَيه، وقال: «وَانَبِيَّاهُ، وَاخَليلاهُ، وَاصَفِيَّاهُ».
[حسن] - [رواه أحمد]

‘Ā’ishah (may Allah be pleased with her) reported: Abu Bakr entered upon the Messenger of Allah (may Allah's peace and blessings be upon him) after his death. He ‎put his mouth between his eyes, put his hands on his temples, and said: "O my Prophet, O my ‎close friend, O my bosom friend."
Hasan/Sound. - [Ahmad]

Explanation

After the death of the Prophet (may Allah's peace and blessings be upon him) Abu Bakr (may ‎Allah be pleased with him) entered the room in which he was, kissed him between his eyes, put his hands on ‎his temples, and said: ‎‎"O my Prophet, O my close friend, O my bosom friend." In other words, Abu Bakr (may Allah be ‎pleased with him) was sorrowing and grieving over the Prophet's death, describing him as his sincere friend, whom he loved ‎most of all and favored over all people, including himself.‎ What Abu Bakr (may Allah be pleased with him) did here is known as lamentation, which is allowable as long as the heart harbors no objection or impatience to the affliction. Also, the voice must not be loud, unlike that of the women who shriek and keen (the well-‎known chanting of women when they wail and lament a dead person). ‎