Ubada bin Samit reported,
The Prophet ﷺ said, "Paradise has one hundred levels. Between each two levels is a distance of one hundred years." Affan said, "Like the distance between the sky and the earth. Firdaws is its highest level, from it flow the four rivers, and the Throne is above it. So when you ask Allah 
, ask Him for Firdaws."
Ahmad 22695
Sahihحدثنا يزيد قال حدثنا همام بن يحيى حدثنا عفان حدثنا همام حدثنا زيد بن أسلم عن عطاء بن يسار
عن عبادة بن الصامت
عن النبي ﷺ قال الجنة مائة درجة ما بين كل درجتين مسيرة مائة عام وقال عفان كما بين السماء إلى الأرض والفردوس أعلاها درجة ومنها تخرج الأنهار الأربعة والعرش من فوقها وإذا سألتم الله تبارك وتعالى فاسألوه الفردوس
Its isnad is sahih, and it is found in Tirmidhi 4/675, no. 2531, through three routes. Regarding one of them, he said it is hasan ghareeb, no. 2529. Regarding the second, no. 2530, he noted that Ataa did not hear from Muaz. As for the third, he made no comment on it, from Ataa from Ubadah. He then narrated it through a route similar to Ahmad's, and perhaps he made no comment on it because of the well-known standing of its narrators.Nasai narrated it, 1/19, no. 2131, from Abu Saeed, and Ibn Majah, 6/1448, no. 4331, and Hakim graded it sahih, 1/80, and Dhahabi concurred with him. The disagreement over the obligatory nature of Witr has already been discussed at the hadith, "Allah has increased you a prayer..." in 7910.