What should a Muslim say in praise of another Muslim?

The Prophet ﷺ said: “If any of you must praise his friend (provided he knows this about him), he should use these words: “I think that this person is such-and-such (for example: pious, righteous, a scholar who acts on his knowledge, honest, etc.), and Allah the Most High is the one who will take account of him; I do not declare anyone pure before Allah the Most High.”

1. Sahih al-Bukhari, no. (2662), Sahih Muslim, no. (3000), the wording is his, Sunan Abi Dawud, no. (4805), Sunan Ibn Majah, no. (3744).