Is it true that the Prophet, peace and blessings be upon him, said to Mu'adh: (Do not weep, O Mu'adh, for weeping, or indeed weeping, is from Satan)? And how can this be reconciled with Jacob's sorrow and the Messenger's sorrow over his son Ibrahim?

The Hadith (definition) cited by the questioner from Mu'adh ibn Jabal is authentic, and its apparent meaning indicates a prohibition of crying. However, after gathering all the evidence, it becomes clear that the forbidden crying is that which involves raising one's voice, wailing, or expressing discontent with God's decree.

The Prophet, peace and blessings be upon him, cried upon the death of his son Ibrahim and his granddaughter's son, and when he learned of the martyrdom of Ja'far, Zayd, and Abdullah ibn Rawahah, and upon the death of Uthman ibn Maz'un, and he said: "The eye sheds tears, and the heart grieves, but we only say what pleases our Lord."

The Prophet, peace and blessings be upon him, clarified that crying not accompanied by discontent or wailing is a mercy, and that what is forbidden is "two foolish, wicked sounds: a sound at a calamity, scratching faces, tearing garments, and a devil's wail."

Scholars have unanimously agreed on the permissibility of crying as long as it is not accompanied by lamentation, wailing, tearing of garments, or expressing discontent with God's decree.