Salaat al-Kusoof (Prayer of lunar Eclipse ) is a Sunnah mu’akkadah (a confirmed Sunnah) according to the consensus of the scholars. The daleel (evidence) for this is the Sunnah reported from the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him)

Eclipses are signs from Allaah by means of which Allaah makes His slaves afraid. Allaah says (interpretation of the meaning):“And We sent not the signs except to warn, and to make them afraid (of destruction)” [al-Israa’ 17:39]

When the sun was eclipsed at the time of the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him), he went out rushing nervously to the mosque, dragging his cloak behind him, and led the people in prayer. He told them that the eclipse was one of the signs of Allaah, with which Allaah makes His slaves afraid, and that it may be the cause of punishment coming upon the people. He commanded them to do that which could prevent the punishment, so he commanded them to pray when an eclipse happens, and to make du’aa’, seek His forgiveness, give charity, free slaves and do other righteous deeds so that the punishment would go away and not befall the people. So the eclipse is a reminder to people, making them afraid so that they will turn back to Allaah and pay attention to Him.

During the Jaahiliyyah, people used to believe that eclipses happened to mark the birth or death of a great person, but the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) declared this belief to be false and explained the divine wisdom behind the occurrence of eclipses:

First ever geographically detailed map of Mecca as it was during the lifetime of the Prophet Muhammad (PBUH)

Imaam al-Bukhaari and Muslim narrated that Ibn Mas’ood al-Ansaari said: “the sun was eclipsed the day Ibraaheem the son of the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) died, and the people said, ‘The sun is eclipsed because of the death of Ibraaheem.’ The Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) said: ‘The sun and the moon are two signs from Allaah, and they do not become eclipsed for the death or the birth of anyone. If you see that, hasten to remember Allaah and to pray.’”