The call for Tabuk came in fierce heat, for a long journey against a strong enemy. Ka'b ibn Malik was ready and able — but saying 'today, tomorrow', he let the army leave without him.
❝ HadithWhen the Messenger of Allah ﷺ returned, those who had stayed behind came with excuses, and he accepted what they said outwardly and sought forgiveness for them. Ka'b said: I thought, I may please him today with a lie, but Allah will soon make him angry with me. So I told the truth: I had no excuse. Then the Messenger of Allah ﷺ forbade people to speak to the three of us. The earth, for all its breadth, felt narrow to me. After fifty nights, one dawn someone shouted from the hill of Sal': O Ka'b ibn Malik, rejoice! Allah has accepted our repentance.Sahih al-Bukhari 4418 · Sahih
وَعَلَى ٱلثَّلَٰثَةِ ٱلَّذِينَ خُلِّفُواْ حَتَّىٰٓ إِذَا ضَاقَتۡ عَلَيۡهِمُ ٱلۡأَرۡضُ بِمَا رَحُبَتۡ وَضَاقَتۡ عَلَيۡهِمۡ أَنفُسُهُمۡ وَظَنُّوٓاْ أَن لَّا مَلۡجَأَ مِنَ ٱللَّهِ إِلَّآ إِلَيۡهِ ثُمَّ تَابَ عَلَيۡهِمۡ لِيَتُوبُوٓاْۚ إِنَّ ٱللَّهَ هُوَ ٱلتَّوَّابُ ٱلرَّحِيمُ
“And [He also forgave] the three who were left alone [i.e., boycotted, regretting their error] to the point that the earth closed in on them in spite of its vastness and their souls confined [i.e., anguished] them and they were certain that there is no refuge from Allāh except in Him. Then He turned to them so they could repent. Indeed, Allāh is the Accepting of Repentance, the Merciful.”
Surah At-Tawbah 9:118
Translation: Saheeh International (QuranEnc)
Ka'b used to say that of all the blessings Allah gave him after Islam, the greatest was telling the truth to the Messenger of Allah ﷺ that day; had he lied, he would have perished like those who made excuses.