It was explained earlier that if it has entered 1 Dzulhijjah, then it is not permissible for the qurban shohibul to cut hair and nails until the qurban animal is slaughtered as stated here. What if someone did it on purpose? Is the sacrifice (udhiyah) still valid?
Shaykh Muhammad bin Sholih Al 'Utsaimin rahimahullah explained,
، لة لكنه اصياً ا ا اشتهر العوام ا الإنسان ام ال لا له
"Na'am, The sacrifices made are still accepted (legitimate) but those who violate them hit by sin. While the perception of ordinary people who consider their qurban to be invalid if someone shaves head hair, cuts nails or plucks body hair in the first ten days of Dhulhijjah, then this is an incorrect assumption. Because actually there is no connection between the validity of the sacrifice and revoking these three things." (Syarhul Mumthi', 7:533)
@ Sakan 27 Jami'ah Malik Su'ud, Riyadh, KSA, 29/11/1433 H