Imaam Ahmad (rahimahullaah) used to say:

إذا مَا خَلَوتَ الدَّهْرَ يومًا فلا تَقُلْ *** خَلوتُ ولكن قُلْ عليَّ رَقِيبُ
ولا تحْسَـبنَّ اللـهَ يَغْفِـل ساعَـةً *** ولا أنّ مَا تخفَي عَليه يَغِيبُ
ألم تر أنّ اليومَ أسْرعُ ذاهِبٍ *** و أنّ غداً للناظِرين قَريبُ

“If you spend a day in solitude, do not say
I spent it in solitude, but say over me is a Watcher

And do not think that Allaah is oblivious for even an hour
or that what you try to hide from Him is unseen

Do you not see that today is leaving fast
And that tomorrow for the onlooker is indeed near?

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It is said that Maalik ibn Dinaar (rahimahullaah) once went to the graveyards and said:

أتيتُ القبور فناديتها *** أين المعَظَّمُ و أين المُحْتَقِرُ
و أين المُلَبِّى إذا ما دَعا *** و أين العَزِيز إذا ما افتَخِرُ
و أين المُدَلّ بِسُلْطَانِه *** و أين القوِِّىُّ أذا ما قدَرُ

“I approached the graves and called out to it
‘Where is the exalted one and where is the debased?

Where is the caller when he did call out
And where is the powerful when he did boast

Where is the one who was arrogant in his reign
And where is the strong one when he did gain power?’

فسمع صوتاً يقول له

تفَانوا جَمِيعاً فما مُخْبر *** و مَاتوا جَميعاً و ماتَ الخبرُ
تروح و تغدو بنات الثرى *** فَتَمْحُو مَحاسِنَ تلك الصُوَر
فيا سائِلى عن أناسٍ قد مَضَوا *** أمَّا لك فِيما تَرى مُعْتَبِر

So he heard a voice say:

All have fleeted, so there is no informer
And all have died and so did the news

The daughters of the rich come and go
So they wipe away the beauty of such a picture

So O questioner of a people who have gone by
Is there not in what you see, a lesson indeed?

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Al-Abrash said (and this is also attributed to ‘Abdullaah ibn al-Mubaarak and ash-Shaafi’ee, rahimahumullah):

تعلَّم فليس المرءُ يولَد عالَماً *** و ليس أخو عِلمٍ كَمَن هو جاهِلُ
و أنّ كبيرَ القومِ لا عِلم عندَه *** صغيرٌ إذا التفَّت عليه المَحافِلُ

Learn! For a person is not born a scholar
And the companion of knowledge is not like the ignorant

And no knowledge is insignificant for the noble one of a nation
If the masses themselves turn towards it

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Al-Muntasir ibn Bilaal al-Ansaari:

و لن يَهلِكَ الإنسانَ إلا إذا أتى *** مِن الأمر ما لم يرضهُ نصحاؤه
و أقلل إذا ما قلتَ قولاً, فإنه *** إذا قلَّ قول المرء قلَّ خطـاؤه

A person can never be ruined unless he approaches
A matter which is not pleasing to his sincere advisors

And lessen your speech when you do speak,
For when the speech of a person is lessened, their mistakes are lessened
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[Most of these abyaat have been taken from the book ‘Rawdhatul-‘Uqalaa wa Nuzhat al-Fudhalaa’ by Ibn Hibbaan]