And We made an appointment with Moses for thirty nights (and days to pray in solitude) which We supplemented with another ten (to receive the Law) so that the period appointed by his Lord came to be full forty nights (and days). (When leaving) Moses said to his brother Aaron, ‘Act for me (taking my place) in my absence among my people and reform and manage (them) well and do not follow the way of those who create disorder.’ (7:142)

Commentary
NUMBER 40: Prophet Moses is one of the most talked about prophets in the Quran with his name coming up a total of 136 times – from Surah al-Baqarah (2) all the way to Surah al-Aʿlā (87). His name is mentioned alongside other Prophets, such as Ibrāhīm, ʿIsā, Hārūn and also alongside his arch nemesis, the Pharaoh (Firʿawn).
In this particular verse, a “unique” thing is seen in which Allah first had an “appointment” with Mūsā for 30 nights had and then all of a sudden , He added 10 more nights to the event – making it 40 nights in total. In other verses, Allah mentions the 40 nights straight up, but here he makes us do some math – but WHY? 
The opinion which our scholars mention and is found in the aḥādīth from the Ahlulbayt points to the fact that Allah had always decreed this sojourn of Mūsā to be for 40 days and nights however in order to test his community, the Children of Israʾīl, he was first invited for 30 days and then the stay period was prolonged by another 10 days being added to it – this was one way for the imposters and hypocrites among the tribes of Isrāʾīl to be differentiated from the true believers.
Ultimately, faith does not come easy and it is by testing – whether it be Prophet Mūsā and his 40 days in the spiritual retreat in communion with Allah, Prophet Muḥammad ] and his proclamation on the 18th of Dhū al-Ḥijjah at the event of Ghadir al-Khumm, or the major occultation (ghaybah) of Imam al-Mahdi Q for the past 1,200 years. These are all ways that Allah tests the believers in their faith – and as we see in all accounts, there are winners and losers.
The number “40” is unique in Islamic teachings and we see its repetition quite often in the Quran, aḥādīth and the supplications:
1. Prophet Nūḥ faced 40 days of rain as a test for the disbelievers;
2. Prophet Muḥammad announced his Prophethood at the age of 40;
3. Once a person reaches 40, they are at their ‘peak’ according to the Quran;
…and the list goes on and on.
In summary, this verse gives us many life lessons which if we study the time of Mūsā and what he went through, we will be able to find some parallels in our lives – albeit at a much smaller scale.
Point of Reflection
In order for us to be successful in our lives, in whatever we want to do, we must ensure that we have the prerequisites and that we are constantly working on spiritual self-building to prove to those around us and to Allah that we are ready to tackle the greatest of challenges – and whatever life may put in our paths.