The Quran

۞ وَأَقسَموا بِاللَّهِ جَهدَ أَيمانِهِم لَئِن أَمَرتَهُم لَيَخرُجُنَّ ۖ قُل لا تُقسِموا ۖ طاعَةٌ مَعروفَةٌ ۚ إِنَّ اللَّهَ خَبيرٌ بِما تَعمَلونَ

Muhammad Asad

Now [as for those half-hearted ones,] they do swear by God with their most solemn oaths that if thou [O Apostle] shouldst ever bid them to do so, they would most certainly go forth [and sacrifice themselves].¹ Say: "Swear not! Reasonable compliance [with God's message is all that is required of you].² Verily, God is aware of all that you do!"

This is an allusion to the ephemeral, self-deceiving enthusiasms of the half-hearted and their supposed readiness for "self-sacrifice", contrasting with their obvious reluctance to live up to the message of the Qur'an in their day-to-day concerns.
This elliptic phrase alludes to the principle - repeatedly stressed in the Qur'an - that God does not burden man with more than he can easily bear.