Interestingly I couldn't find this quote of Mujahid -so far- in any tafssir related to the two Verses quoted in sahih al-Bukhari.
In his fath al-Bari ibn Hajar al-'Asqalani quoted a chain:
قوله : وقال مجاهد استوى : علا على العرش ) وصله الفريابي عن ورقاء عن ابن أبي نجيح عنه
Note that this is my own translation take it with the encessary care, I'll only quote the narrator chain:
Al-Firyabi محمد بن يوسف الفريابي has narrated (the original wording actually is "connected") it from Warqaa' ورقاء from ibn abi Najih ابن أبي نجيح from him (meaning Mujahid).
Muhammad ibn Yusuf al-Firyabi is a teacher of imam al-Bukhari (for details see in siyar a'laam an-Nubalaa') .
Warqa' is Warqa' ibn 'Omar ورقاء بن عمر ibn Kulayb is also considered as a hafidh and trustworthy (see siya a'laam an-Nubalaa').
Abduallah ibn abi Najih actually is a trustworthy narrator and mufassir who has narrated from 'Ata' and Mujahid and was teacher of Sufyan ibn 'Oyayna and Sufyan at-Thawri some considered him from among the Mu'tazila, but all agree that when it comes to tafssir he is trustworthy (Summary from siyar a'laam an-Nubalaa' of a-Dhahabi)
The same sanad can also be found in other commentaries such as irshaad as-sari of al-Qustalani and 'Omdat al-Qari of al-'Ayni.