Allah ordered in Quran : Take and follow whatsoever the Messenger of Allah tells you, and abstain from whatsoever he forbids you. (Sura 59, verse 7).
This Quranic order is very clear and obligatory on every believer. Lot of very important matters in Islam were not mentioned in Quran but ordered by the Prophet ( SAWA) hence became obligatory on all Muslims like the number of Rak'ats in every Salaah (Prayer). All Muslims perform Morning Prayers in two Rak'ats, Dhuhr in four Rak'ats, Asr in four Rak'ats, Maghrib in three Rak'ats and Isha in four Rak'ats just because the Prophet told them and Allah's order in Quran (Sura 59, verse 7).
Allah says in Quran as well : He who obeys the Prophet has indeed obeyed Allah. (Sura 4, verse 80).
The Prophet Muhammad (SAWA) has clearly and repeatedly mentioned that his successors will be 12 Imams. This has been mentioned in most authentic Hadeeths narrated in both Shia and Sunni main references like:
1. Saheeh Al-Bukhari , Hadeeth number 6682.
2. Al-Tareekh Al-Kabeer by Al-Bukhari, Volume 8, page 410.
3. Saheeh Muslim , Hadeeth number 3393 and 3394 and 3395 and 3396 and 3397 and 3398.
4. Musnad Ahmad ibn Hanbal in 36 Hadeeths ; 3593 and 3665 and 9875 and 19884 and 19887 and 19892 and 19901 and 19914 and 19920 and 19920 and 19925 etc.
5. Tirmithi (Tirmizi) Hadeeth number 2149.
6. Sunan Abi Dawood , Hadeeth 3731 and 3732.
7. Mustard Al Al-Saheehain by Al-Haakim Al-Nisabouri, Hadeeths 6586 and 6589 and 8529.
8. Al-Albani Nasser Al-Deen, Kuruba Takhreej Al-Hadeeth, Hadeeth 376 and 1075 and 3598 and 3599.
9. Ibn Hajar Al-Asqalani in Ta'jeel Al-Manfa'ah, page 538, and in Al-Mataalib Al-'Aaliyah , 2140 and 4608.
10. Al-Bayhaqi in Dala'il Al-Nubowwah, Hadeeth 2589 and 2888 and 2890 and 2895
And many other Sunni books.