SURAH AL-E-IMRAN: 166-168

TRANSLATION:

[3:166]
Whatever you suffered on the day when the two troops faced each other was by the will of Allah, and in order to know the believers,

[3:167]
and in order to know those who are hypocrites. It was said to them, "Come on, fight in the way of Allah, or defend". They said, "Had we known it to be a fight we would have certainly followed you". {57} That day, they were nearer to disbelief than they were to Belief. They utter from their mouths what is not in their hearts.  {58} Allah knows well what they conceal.

[3:168]
Those who said about their brethren, while themselves sitting, "Had they obeyed us, they would have not been killed". Say, "Then avert death from yourselves if you are true."

INTERPRETATION:

{57} What they meant was that if it was a battle of equal forces they would have joined the battle but in this battle Muslims were vastly outnumbered by the non-believers. As the numbers of non-Muslims was more than three times the number of Muslims this was not war, it was suicide, therefore, they could not join it.

{58} Meaning that even though they claim that if it was a battle of equal forces they would have joined it, but this was just an excuse. In their hearts, they had already decided that they were not going to fight on the side of the Muslims even if it was an equal contest.