Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
ahmad:10069 – ʿAbd al-Raḥman > Ḥammād > Muḥammad > Abū Hurayrah > Abū al-Qāsim ﷺ > Amā Yakhshá Aḥadukum Idhā Rafaʿ Raʾsah And al-Imām Sājid

[Machine] I heard Abu Al-Qasim ﷺ saying, "Doesn't anyone fear that when he raises his head and the Imam is in prostration, Allah may turn his head into the head of a donkey?"  

أحمد:١٠٠٦٩ – حَدَّثَنَا عَبْدُ الرَّحْمَنِ حَدَّثَنَا حَمَّادٌ عَنْ مُحَمَّدٍ قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ يَقُولُ

سَمِعْتُ أَبَا الْقَاسِمِ ﷺ يَقُولُ أَمَا يَخْشَى أَحَدُكُمْ إِذَا رَفَعَ رَأْسَهُ وَالْإِمَامُ سَاجِدٌ أَنْ يَجْعَلَ اللهُ رَأْسَهُ رَأْسَ حِمَارٍ  

See similar narrations below:

Collected by Suyūṭī
suyuti:4247a –

Abu Hurairah reported the Messenger of Allah ﷺ as saying; Does he who raises his head while the Imam is prostrating not fear that Allah may change his head into a donkey’s or his face into a donkey’s face. (Using translation from Abū Dāʾūd 623)   

السيوطي:٤٢٤٧a –

"أما يخشَى أحدُكم إذا رفعَ رأسَهُ قبلَ الإِمام أن يجعلَ اللهُ رأسَهُ رأسَ حمارٍ، أوْ يجعلَ اللهُ صُورته صورةَ حمارٍ".  

[حم] أحمد [ش] ابن أبى شيبة [خ] البخاري [م] مسلم [د] أبو داود [ت] الترمذي [ن] النسائي [هـ] ابن ماجة عن أبي هريرة (وفي رواية لابن حبان: أن يحول الله رأسه رأس كلبٍ)