The Quran

وَمِنهُم مَن يَقولُ ائذَن لي وَلا تَفتِنّي ۚ أَلا فِي الفِتنَةِ سَقَطوا ۗ وَإِنَّ جَهَنَّمَ لَمُحيطَةٌ بِالكافِرينَ

Muhammad Asad

And among them there was [many a one] who said,¹ "Grant me permission [to remain at home], and do not put me to too hard a test!" Oh, verily, [by making such a request] they had [already failed in their test and] succumbed to a temptation to evil:²and, behold, hell will indeed encompass all who refuse to acknowledge the truth!

I.e., at the time when the Prophet was making preparations for the campaign.
See verses 44 and 45 above. It is to be noted that both the verbal form la taftinni (rendered by me as "do not put me to too hard a test") and the noun fitnah have the same root, comprising a great complex of meanings: e.g., test, trial, affliction. temptation to evil, seduction, persecution, oppression, discord, civil strife, etc. (cf. surah 8, note 25). Since it is impossible in any language but Arabic to reproduce all these many shades of meaning in a single expression, the rendering of the term fitnah must necessarily vary in accordance with the context in which it is used.