It is mentioned in Bahaare Shariat Sadr al Shariah Shaykh Mufti Amjad 'Ali al A'azami 'alaihi al rahmah as above. Raising of the toes in sajdah (postration) If both feet are raised from the ground or just the tips of the toes are placed on the ground during the sajdah, and the bottom part of the toe is not touching the ground the prayer will not be valid. If a few toes of the feet including the bottom parts touch the ground but most of them are left raised, then in this state the salah will be highly disliked (Makrooh Tahrimi) and it must be repeated (Wajib ul Ia’adah). Ashi’at ul Luma’at V1 page 394 Dur ul Mukhtaar ma’ Rad ul Muhtaar V1 page 313 “to place one toe (on the ground) is a condition (for the acceptance of the Prayer)” Dur ul Mukhtaar ma’ Rad ul Muhtaar V1 page 351 “it is fard to place one of the toes (on the ground) pointing to the Qiblah (direction of the Ka’abah in Makkah) the general people are not aware of this” Fatawaa Fayd ur Rasool V1 page 250 Fatawaa Barkaatiyah page 95 May Allah grant the Muslim the ability to learn how to pray correctly according to the Islamic Shariah Aameen Ya Rabb al 'Aalameen
basically, tips of at least 3 toes of each foot must point to qibla, which is only possible if toes are laid as described above. If I remember correctly, it's mustahab for all 10 toe-tips to be pointing to qibla during sajda.
in sajda, it is wajib for 3 fingers of each foot to be touching the ground. not just the tips of the toes, but the part that our urdu speaking fuqaha called the peyt. a brother told me that this is known as the plantar surface of the toes.