Musnad Ahmad

It is narrated from Hazrat Abu Musa (may Allah be pleased with him) that once we performed the Maghrib prayer with the Noble Prophet (peace and blessings be upon him), then we thought to wait a little while and perform the Isha prayer with the Noble Prophet (peace and blessings be upon him) as well. So we kept waiting. When the Noble Prophet (peace and blessings be upon him) arrived, he asked, "Are you still here at this time?" We replied, "Yes, O Messenger of Allah (peace and blessings be upon him)! We thought to perform the Isha prayer with you." The Noble Prophet (peace and blessings be upon him) said, "Very good." Then he raised his head towards the sky, and he would often raise his head and look towards the sky, and said, "The stars are a sign of security for the sky; when the stars disappear, then that calamity will come upon the sky which has been promised. And I am a sign of security for my companions (may Allah be pleased with them); when I depart, then that calamity will come upon my companions (may Allah be pleased with them) which has been promised to them. And my companions (may Allah be pleased with them) are a sign of security for my Ummah; when they also depart, then that trial will come upon my Ummah which has been promised to them."