لَا صَغِيرَةَ إِذَا قَابَلَكَ عَدْلُهُ، وَلَا كَبيرَةَ إِذَا وَاجَهَكَ فَضْلُهُ.
There is no minor sin when His justice confronts you; and there is no major sin when His grace confronts you. 
Should Allāh’s attribute of justice (ʿadl) be manifested on earth, dealing with us according to the strict criteria of such equity, the smallest of sins (ṣaghīra) would be treated as an enormity (kabīra). In truth, a minute action of disobedience to such a majestic and great Lord and Benefactor, whose grandeur is infinite and bounties are incalculable, justifies the application of complete punishment upon the disobedient one.
If, on the other hand, His grace is manifested, then our greatest sins wane into oblivion, after becoming insignificant due to His magnanimous forgiveness.
إِذَا وَقَعَ مِنْكَ ذَنْبٌ .. فَلَا يَكُنْ سَبَبًا لِيَأْسِكَ مِنْ حُصُولِ الْاِسْتِقَامَةِ مَعَ رَبِّكَ، فَقَدْ يَكُونُ ذٰلِكَ آخِرَ ذَنْبٍ قُدِّرَ عَلَيْكَ.
When you commit a sin, let it not be a reason for your despairing of attaining to righteousness before your Lord, for that might be the last decreed for you. 
When one commits some sin, as the result of human nature, one should never allow the sin to bring about the slightest change in one’s determination to remain steadfast upon the religion. Do not permit it to slacken one’s bond with Allāh Most High. Do not be deceived by the thought “I am unable to remain steadfast on the religion, hence my sinning; therefore I might as well give up!” Never think that steadfastness is impossible in the aftermath of committing a sin. Rather, one must hope—as the aphorism suggests—that maybe Allāh will not destine another sin for one.
With this hope, one must be very careful to not become audacious in sin. Indeed, a sin committed spontaneously due to temporary human weakness is not in conflict with steadfastness upon the religion; however, persistence in sins without constant recourse to repentance, whereby one engages in a prolonged and deliberate commission of sins, is a complete negation of steadfastness.
رُبَّمَا دَخَلَ الرِّيَاءُ عَلَيْكَ مِنْ حَيْثُ لَا يَنْظُرُ الْخَلْقُ إِلَيْكَ.
Sometimes ostentation penetrates you in such a way that no one notices it. 
Ostentation is to render an act of worship or some other virtuous deed with the motive of creating an impression on others, in order that they consider one a pious and saintly person.
If a person commences an act of worship in the presence of people, or in the absence of people who later arrive, with the sole desire to create the impression of piety—such as performing the ritual prayer in a beautiful manner [which is not one’s normal practice]—then this servant has committed an act of ostentation [which is clear to almost anyone].
Sometimes, however, a person renders an act in private; yet, notwithstanding this privacy, ostentation enters his heart. This kind of ostentation is very subtle. The sign of this type of ostentation is a man’s desire to be honored when he meets others: that he be appointed to a position of prominence and leadership, with others serving him. When he is honored, he becomes elated. When such honor is not forthcoming from people, he is stung with surprise. This attitude indicates that the person’s efforts and deeds are motivated by the desire for name and fame, and to gain service from people.