What is the ruling on saying, 'We know nothing but good' (La na'lam illa khayran) regarding the deceased?

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Saying, 'We know nothing but good,' is permissible only for one whose evil deeds are unknown, to avoid bearing false witness. This is based on a tradition where the Prophet (peace be upon him) taught this phrase, and when asked what to say if no good is known, he commanded not to say anything except what is known. This practice is legislated because when the deceased was praised well, the Prophet said, 'It is incumbent (Wajabat),' and when the deceased was spoken ill of, he also said, 'It is incumbent,' adding that some people bear witness for others. Another Hadith states that when two of the nearest neighbors testify to good about a deceased Muslim, Allah accepts their testimony for what they knew and forgives what He knows.