SB 1.7.55

sūta uvāca
arjunaḥ sahasājñāya
 harer hārdam athāsinā
maṇiṁ jahāra mūrdhanyaṁ
 dvijasya saha-mūrdhajam
Synonyms: 
sūtaḥ — Sūta Gosvāmī; uvāca — said; arjunaḥ — Arjuna; sahasā — just at that time; ājñāya — knowing it; hareḥ — of the Lord; hārdam — motive; atha — thus; asinā — by the sword; maṇim — the jewel; jahāra — separated; mūrdhanyam — on the head; dvijasya — of the twice-born; saha — with; mūrdhajam — hairs.
Translation: 
Sūta Gosvāmī said: Just then Arjuna could understand the motive of the Lord by His equivocal orders, and thus with his sword he severed both hair and jewel from the head of Aśvatthāmā.
Purport: 

Contradictory orders of different persons are impossible to carry out. Therefore a compromise was selected by Arjuna by his sharp intelligence, and he separated the jewel from the head of Aśvatthāmā. This was as good as cutting off his head, and yet his life was saved for all practical purposes. Here Aśvatthāmā is indicated as twice-born. Certainly he was twice-born, but he fell down from his position, and therefore he was properly punished.