During our lifetime we encounter usually a couple of happy and sad incidents like that of the death of a beloved person or that of a relative.
And as we are informed from the sunnah:
“The Messenger of Allah (ﷺ) said: ‘Whoever consoles a person stricken by calamity will have a reward equal to his.’” (See for example in Sunan ibn Majah)
The Messenger of Allah said: "Whoever consoles a bereaved mother, he will be clothes with a Burd in Paradise." (Jami' at-Tirmidhi)
In Muslim societies many expressions of condolence have been established mainly based on custom and partly based on the Muslim sources. And as many of us don't know which of these have a backup in the Qur'an and sunnah and which not (and I count my self to these). I thought it would be good to ask the following:
Which expressions of condolence have a backup in the Qur'an or sunnah and what is accordingly the adequate answer?
An additional qualification of proposed hadith narrations would be a bonus!
Beside this what expressions have scholars recommended or consider as permissible?