Here is a blog post by IslamicSalvation, which can be found here:
باب الجمع بين الصلاتين
Chapter on the combining of prayers
محمد بن يحيى، عن أحمد بن محمد، عن علي بن الحكم، عن عبدالله بن بكير، عن زرارة، عن أبي عبدالله (عليه السلام) قال: صلى رسول الله (صلى الله عليه وآله) بالناس الظهر والعصر حين زالت الشمس في جماعة من غير علة وصلى بهم المغرب والعشاء الآخرة قبل سقوط الشفق من غير علة في جماعة وإنما فعل رسول الله (صلى الله عليه وآله) ليتسع الوقت على امته (موثق)
1. Muhammad bin Yahya from Ahmad bin Muhammad from Ali bin Hakam from Abdallah bin Bukayr from Zurara (bin A’yan) from Abi Abdillah (a.s) who said : The messenger of Allah (s.a.w.w) prayed with the people - the Dhuhr and the Asr (one immediately after the other) - when the sun had crossed the meridian (Zawal), in congregation, and without there being a necessitating cause (for it), and He (s.a.w.w) also prayed with them - the Maghrib and the Isha (one immediately after the other) - before the (complete) disappearance of the redness of the horizon (after sun set), without there being a necessitating cause, and (this too) in congregation, and verily the messenger of Allah (s.a.w.w) did so in order to ease the burden of time upon his Ummah.
(Muwathaq due to Abdallah bin Bukayr)
NOTE : The Muwathaqa of Zurara is clear in the Rukhsah (dispensation) for combining, due to it being practised by the messenger of Allah (s.a.w.w) as reported by the Imam, and that too without in congregation, without there being a specific cause, and with the intention of easing the burden upon the Ummah.
Which hadith book does this hadith come from? Thanks.