He thought that hastening to depart from Mina early meant leaving it even if one has not stoned the Jamaraat
Question

                We hastened to leave Hajj early and departed from Mina before sunset, so that we would not have to stay until the third day. After leaving Mina we came back and stoned the jamaraat after sunset. Is what we did permissible? Is it essential for the one who wants to leave early to stone the jamaraat before sunset on the second day, or does the stoning have nothing to do with it, and is leaving before sunset what matters? What should we do, if we have made a mistake?.
Praise be to Allah.
    This understanding of the word ta’ajjul (hastening) is not 
    correct, especially since for a pilgrim to “hasten” means that he leaves 
    Mina before sunset on the second of the days of al-tashreeq, after having 
    stoned the jamaraat. Simply leaving Mina before sunset does not count as 
    “hastening” if he has not stoned the jamaraat, especially if he comes back 
    after sunset to stone them. 

    The scholars of the Standing Committee said: 

    If the pilgrim stones the Jamaraat on the twelfth day after 
    the sun has passed its zenith, then departs to Makkah or elsewhere before 
    the sun sets, he does not have to stone the Jamaraat on the thirteenth, and 
    it is not prescribed for him to do so.

    Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah 
    wa’l-Ifta’, 11/274. 

    Commenting on the verse (interpretation of the meaning), 
    “And remember Allaah during the appointed Days. But whosoever hastens to 
    leave in two days, there is no sin on him and whosoever stays on, there is 
    no sin on him, if his aim is to do good and obey Allaah (fear Him)” 
    [al-Baqarah 2:203], the scholars of the Standing Committee said: 

    With regard to the meaning of what is mentioned in the verse: 
    whoever hastens to depart from Mina after spending two nights there 
    following the Day of Sacrifice, and after stoning the three jamaraat on the 
    eleventh and twelfth, there is no sin on him and he does not have to offer a 
    (compensatory) sacrifice, because he has done what is required of him. 
    Whoever tarries in Mina and spends the night of the thirteenth there and 
    stones the three jamaraat on the thirteenth, there is no sin on him either, 
    rather his staying in Mina on this night and stoning the jamaraat on this 
    day is better and brings a greater reward, because the Prophet  (peace 
    and blessings of Allaah be upon him) did that.

    Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah 
    wa’l-Ifta’, 11/267. 

    Shaykh Saalih al-Fawzaan said: 
The minimum length of 
    stay in Mina during the days of Hajj is the eleventh and twelfth – the two 
    days after Eid – and this is what is meant by hastening to leave. But it is 
    better to stay on the thirteenth too, which is what is meant by staying on. 
    Allaah says (interpretation of the meaning): 

    “But whosoever hastens to leave in two days, there is no 
    sin on him and whosoever stays on, there is no sin on him, if his aim is to 
    do good and obey Allaah (fear Him)” 

    [al-Baqarah 2:203]

    Hastening to leave means going away from Mina after stoning 
    the Jamaraat on the twelfth day after the sun has passed its zenith and 
    before the sun sets. If sunset comes and a person has not left Mina yet, he 
    is obliged to stay there on the night of the thirteenth and stone the 
    jamaraat after Zuhr on the thirteenth. And Allaah knows best. 

    Fatwa no. 16386 

    Shaykh Ibn ‘Uthaymeen was asked: 

    A pilgrim did not stone the jamaraat on the twelfth because 
    he thought that this was part of hastening to leave Mina, and he did not 
    stay overnight in Mina or perform tawaaf al-wadaa’ out of ignorance. 

    He replied: 

    Your Hajj is valid, because you did not omit any pillar or 
    essential part of the Hajj, but you omitted three obligatory duties. The 
    first was staying overnight in Mina on the night of the thirteenth. The 
    second was stoning the jamaraat on the twelfth. The third was performing 
    tawaaf al-wadaa’ (the farewell tawaaf). According to the scholars, if a 
    person omits this tawaaf during Hajj, he has to offer a compensatory 
    sacrifice, to be slaughtered in Makkah and its meat distributed to the poor. 
    But not staying overnight in Mina for one night does not require a 
    compensatory sacrifice. 

    I would like to take this opportunity to draw the attention 
    of my pilgrim brothers to this mistake made by our brother who asked the 
    question, because many pilgrims take the words of Allaah “But whosoever 
    hastens to leave in two days” to mean leaving Mina on the eleventh, so 
    they count the two days as the day of Eid and the eleventh, but that is not 
    how it is, rather this is a misunderstanding, because Allaah says 
    (interpretation of the meaning): 

    “And remember Allaah during the appointed Days. But 
    whosoever hastens to leave in two days, there is no sin on him” 
    

    [al-Baqarah 2:203]

    The appointed days (al-ayyaam al-ma’doodaat) are the 
    ayyaam al-tashreeq, the first of which is the eleventh. Based on this, the 
    words of Allaah “But whosoever hastens to leave in two days” mean two 
    of the days of al-tashreeq, i.e., on the twelfth. We have to make sure that 
    we understand such matters correctly so that we will not make mistakes. 

    Fataawa al-Hajj, question no. 
    40. 

    And Allaah knows best.